Hip joint prosthesis convertible in vivo to a modular prosthesis

ABSTRACT

A nonmodular prosthesis having a retainer for a modular bearing and a nonmodular primary bearing directly molded to the base.

REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.09/174,215, filed Oct. 16, 1998, abnd. and entitled: “Nonmodular JointProsthesis Convertible In Vivo to a Modular Prosthesis”.

BACKGROUND OF THE INVENTION

This invention relates to surgically implantable joint prostheses and,more particularly, to a joint prosthesis that combines advantages ofmodular and nonmodular prostheses.

Artificial joints of the human body, including in particular knee andhip joints, have been available for 50 years or more and have been thesubject of intense development for at least the last 20 years. Theearliest designs provided metal-to-bone or metal-to-metal contactbetween the articulating surfaces of a joint. Friction and wear weresignificantly reduced in subsequent designs by the introduction ofultra-high molecular weight polyethylene (UHMWPE) as a load-bearingsurface. For example, a typical knee joint prosthesis has a tibialcomponent with a polyethylene load-bearing surface in contact with ametal femoral component. Early designs of this type had polyethylenecemented to the tibia, but it is conventional today to secure apolyethylene bearing to a metal base or tray which is anchored in thetibia, typically with the aid of a stem or peg extending into themedullary canal of the tibia.

Numerous methods of securing a polyethylene bearing to a metal base havebeen developed over the years, as exemplified by the following patents:

Patent No. Inventor Issue Date 4,205,400 Shen et al. June 3, 19804,217,666 Averill Aug. 19, 1980 4,501,031 McDaniel et al. Feb. 26 19854,714,474 Brooks et al. Dec. 22, 1987 4,795,468 Hodorek et al. Jan. 3,1989 4,822,362 Walker et al. Apr. 18, 1989 4,938,769 Shaw Jul. 3, 19904,959,071 Brown et al. Sep. 25, 1990 4,963,152 Hofmann et al. Oct. 16,1990 4,997,445 Hodorek Mar. 5, 1991 5,061,271 Van Zile Oct.29, 19915,080,675 Lawes et al. Jan. 14, 1992 5,108,442 Smith Apr. 28, 19925,330,534 Herrington et al. Jul. 19, 1994

There are two general types of components: modular and nonmodular. Anonmodular prosthesis has a bearing secured to the base duringfabrication in the factory, typically by direct compression molding. Amodular prosthesis has a prefabricated bearing designed to be attachedto the base during surgery.

A modular prosthesis has several advantages over nonmodular prostheses,one of which is that an assortment of different prostheses, i.e.,different base/bearing combinations, can be created in the operatingroom from a small inventory of separate bases and bearings of varioussizes, shapes and other characteristics. With a modular prosthesis, anorthopedic surgeon can implant an appropriate base for the patient andthen fit the patient with several trial bearings in the process ofselecting an appropriate primary bearing to attach to the implantedbase. Modular bearings are often readily removable, and in such casesthey have the further advantage of facilitating revision surgery, whichmay become necessary in cases of traumatic injury or bearing surfacewear, by enabling replacement of the bearing without removing the base.

There are also advantages to a nonmodular prosthesis construction, suchas design simplicity due to the absence of a need for a retainingmechanism for a removable bearing, and relatively low cost. An even moresignificant advantage is that a nonmodular component is virtually immuneto micromotion at the interface between the bearing and the base.

Micromotion is very difficult to avoid with modular components due tothe need for clearances between bearing and base to ensure that they fittogether during assembly in the operating room. A very secure lockingmethod may avoid the problem, but nonmodular fabrication of theprosthesis, e.g., direct compression molding of a bearing onto a base,avoids the issue. Unfortunately, a nonmodular prosthesis has heretoforemade revision surgery more difficult in that the entire prostheticcomponent must be removed and replaced. In addition to the extraoperating time involved and extraction tools required, such as describedin U.S. Pat. No. 4,459,985 to McKay, removal and replacement of the baserequires sacrificing the existing fixation to the bone and hasassociated complications, including possible bone loss or fracture andthe difficulty of reestablishing solid fixation. Nevertheless, it isconventional wisdom regarding a nonmodular prosthesis, as stated in theabove-referenced Smith patent, that “the bearing component cannot bechanged without changing the base component.”

SUMMARY OF THE INVENTION

The present invention combines advantages of modular and nonmodulardesigns with a nonmodular prosthesis that is convertible in vivo to amodular device. The prosthesis includes a base having a retainer for amodular bearing, and has a nonmodular primary bearing secured to thebase. According to one aspect of the invention relating to a knee joint,the retainer includes a raised portion with a cavity in an inwardlyfacing surface thereof, and according to another aspect of the inventionthe retainer is adapted to cooperate with an auxiliary mechanicallocking element to securely retain a modular bearing on the base.

A retainer as that term is used herein is a part of the base that iscapable, alone or in conjunction with an auxiliary element or elements,of retaining a modular bearing in place on the base. It may be formed onthe base as a one-piece or multi-piece retainer.

According to a further aspect of the invention relating to a knee joint,a mechanical release member is mounted on the tibial base in contactwith the primary bearing as a part of the nonmodular tibial prosthesisdesigned to be actuated during revision surgery. Upon actuation, therelease member forces the primary bearing off of the tibial base, afterwhich the primary bearing and release member are removed and replaced bya modular bearing. In some cases, conversion to a modular device wouldbe a desirable option even during the primary surgery. It is routine forsurgeons to check the patent's range of motion before and afterselecting the appropriate size of tibial component and cementing thebase to the tibial plateau. With a nonmodular prosthesis the surgeon haslittle recourse but to dislodge the cemented base and remove it and allthe cement from the tibia if the final range-of-motion check isunsatisfactory. The present invention provides a desirable alternative.

A general object of the present invention is to provide an improvedsurgically implantable joint prosthesis.

Another object of the present invention is to provide advantages of anonmodular joint prosthesis, including the virtual absence ofmicromotion, and yet allow a surgeon performing revision surgery tochange to a modular bearing without disturbing the base member andthereby jeopardizing fixation.

Another object of the invention is to extend the lifetime of anartificial joint.

These and other objects and advantages of the present invention will bemore apparent upon reading the following detailed description of thepreferred embodiment in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a tibial base or tray with a releasemember slidably mounted thereon according to the preferred embodiment ofthe present invention.

FIG. 2 is a perspective view of a complete nonmodular tibial componentaccording to the preferred embodiment of the present invention.

FIG. 3 is a top view of the tibial component of FIG. 2 with one side ofthe bearing cut away to reveal portions of the base and release member.

FIG. 4 is an anterior view of the tibial component of FIG. 2 with oneside of the bearing cut away to reveal portions of the base and releasemember and with the majority of the stem removed for illustrationpurposes.

FIG. 5 is a cross-sectional view of the tibial component taken alonglines 5—5 in FIG. 4.

FIG. 6 is a side view of an extractor together with a sidecross-sectional view of the tibial component of FIG. 2 illustrating thedisengagement of the primary bearing from the base upon actuation of therelease member with the aid of the extractor.

FIG. 7 is a perspective view of a tibial base or tray with a releasemember slidably mounted thereon according to a first alternativeembodiment of the present invention.

FIG. 8 is an anterior view of the tibial component of FIG. 7 with oneside of the primary bearing cut away to reveal portions of the base andrelease member.

FIG. 9 is a perspective view of a tibial base or tray with a releasemember slidably mounted thereon according to a second alternativeembodiment of the present invention.

FIG. 10 is an anterior view of the tibial component of FIG. 9 with oneside of the primary bearing cut away to reveal portions of the base andrelease member.

FIG. 11 is a cross-sectional side view of a hip joint prosthesisaccording to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

For the purpose of promoting an understanding of the principles of theinvention, reference will now be made to the embodiment illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended, such alterations and furthermodifications in the illustrated device, and such further applicationsof the principles of the invention as illustrated therein beingcontemplated as would normally occur to one skilled in the art to whichthe invention relates.

The preferred embodiment of the present invention will be described withprimary reference to a tibial prosthesis for illustration purposes,although the principles of the invention are applicable to other jointprostheses, particularly including artificial hips. With reference toFIG. 1, the preferred embodiment of the present invention asincorporated in a knee joint includes a tibial base or tray 10 and arelease member 12 slidably mounted on the intercondylar portion thereofon a smooth, flat superior surface 14. Release member 12 includes awedge portion 16 and extends between the anterior edge 18 of base 10 anda retaining rail 20 extending superiorly from the posterior edge of thebase. Tibial base 10 includes a pair of posts 22 and 24 integrallyformed on the superior surface 14 of the base at the anterior edgethereof. As described in further detail in U.S. Pat. No. 5,330,534,which is hereby incorporated by reference, the posts and rail aredesigned to cooperate to retain a modular bearing on the base, themodular bearing preferably being preformed of ultra-high molecularweight polyethylene (UHMWPE) with anterior and posterior recesses toreceive the posts and rail, respectively, and with a uniformly flatinferior surface on its intercondylar and medial/lateral portions fordirect contact with the superior surface 14 of the base. The modularbearing is designed to be locked in position with a transverse slide-inlocking bar or clip wedged between the posts and the bearing in opposedgrooves provided therein for that purpose. Modular tibial trays andbearings as generally described above are commercially available fromBiomet Inc., the assignee of the present invention, as components of theMaxim® Total Knee System, which includes various sizes andconfigurations of bases, bearings and stem extensions, as well asfemoral and patellar components, for different patient requirements.

According to the present invention, a modular bearing is not implantedas the primary tibial bearing but instead is secured to the base duringrevision surgery after in vivo removal of a primary bearing which haspreferably been molded over the base and the release member. Such aprimary bearing 26 is shown in FIG. 2 as part of a complete nonmodulartibial component according to the preferred embodiment of thisinvention. UHMWPE is also a suitable material for the primary bearing,which is preferably molded by direct compression molding in aconventional manner.

Release member 12 is preferably formed as a single piece with thethinner, anterior end 28 of wedge 16 integrally joined to an internallythreaded drive block 30 and the thicker, posterior end 32 of the wedgeintegrally joined to a tail 34 which is sized and shaped to partiallyplug a transverse groove or channel 36 formed in the inwardly facingsurface of rail 20. The wedge suitably has an anterior-posterior lengthof approximately 0.5″, a width of approximately 0.5″, and a height ofapproximately 0.25″. The one-piece release member is suitably made ofbiocompatible titanium machined in a conventional manner, or may be castcobalt-chromium. Its surfaces are preferably all smooth.

The shape of the release member is shown in greater detail in FIGS. 3-6,from which it can be appreciated that the molded bearing conforms to theshape of the base and release member, filling all exposed spaces withinthe mold cavity, which is sized with respect to the base to provide askirt 38 which covers the peripheral edge of the base and fills anelongated anterior edge groove 40, two medial/lateral edge grooves 42and two posterior edge grooves 44 in the base as shown in the drawings.The skirt thus cooperates with other portions of the molded bearing, aswill be described, to secure the bearing to the base. To the extent thatany polyethylene wear debris may be produced at the bearing-baseinterface due to micromotion, the skirt also tends to contain suchdebris. The thickness of the skirt adjacent to each such groove,measured in the plane of the superior surface 14 of the tibial base, issuitably approximately 0.020-0.030″. The skirt thickness in that planeis correspondingly greater at the edge portions 41 and 43 of the base,where the edge of the base is cut away from its superior surface to thesuperior surface of a ledge 45 which partially defines the grooves.Preferably the inferior edge 46 of the skirt is trimmed after moldingsuch that it is not flush with the interior surface 47 of the base, aswill be explained, in order to minimize the chances of contact with bonecement during implantation.

Each peripheral edge groove 44 extends from the posterior end of itsrespective edge portion 43 to within 0.1″ of the adjacent edge of rail20. The base has no groove in its posterior edge between the medial andlateral edges of rail 20, as perhaps best shown in FIG. 6. The threadedhole 48 in block 30 is a #12-24 hole and is plugged during the moldingprocess by a set screw (not shown) which is later removed and which may,if desired, extend out of the hole virtually to the mold cavity wall tofacilitate the creation of an access hole 50 for an extractor, as willbe described, in the anterior surface of the bearing. A manual orautomatic trimming step is contemplated to clear any residualpolyethylene out of the desired hole 50, which is suitably circular butmay be square or otherwise shaped. The threaded hole in block 30 may beleft unplugged in the tibial component as implanted.

Transverse groove 36 in rail 20 is partially plugged by tail 34 andpartially exposed in the mold, as perhaps best illustrated in FIGS. 1and 5, and it is filled with polyethylene during the molding process tothe extent so exposed. Anterior grooves 52 and posterior grooves 54 inposts 22 and 24 are entirely exposed and are thus entirely filled withpolyethylene in the preferred embodiment during the molding process.Incidental molding of polyethylene also occurs in the space between therelease member and the superior surface of the base at the medial andlateral edges of the inferior surface of the release member, which edgesare chamfered match the radius of curvature of posts 22 and 24 as bestillustrated in FIG. 4. The chamfered edges preferably extend the entirelength of the release member. The inferior surface of the release memberis otherwise smooth and flat. The medial and lateral edges of thesuperior surface of the wedge at its posterior end 32 may also berounded if desired to maintain a particular bearing thickness at suchpoints. The portions of the molded primary bearing lying in the groovesin the posts and rail, including the portion of groove 36 directlyposterior to tail 34 as well as the portions of groove 36 on either ofthe tail, cooperate with each other and with the molded bearing portionsin peripheral edge grooves 40, 42 and 44 to secure the primary bearingon the tibial base. An example set of groove dimensions suitable forthis purpose is set forth below:

Groove No. Height Width Depth 36 .125″ .8″ .1″ 40 .06″ 2.2″ .04″ 42 .06″.4″ .04″ 44 .06″ .5″ .04″ 52 .125″ .3″ .05″ 54 .1″ .25″ .1

The dimensions in the above table are most suitable for a tibial basehaving a nominal size of 71 mm, ie, a maximum medial/lateral dimensionof 71 mm. The peripheral edge groove widths for other base sizes wouldpreferably vary in approximate proportion to base size.

The tibial component with the bearing as shown in FIG. 2 is designed forcases in which the posterior cruciate ligament is retained. Preferably,the same tibial base can accommodate bearings for cruciate-retainingapplications and bearings for cruciate-substituting applications, wherethe tibial component is subjected to higher moments at the implant-boneinterface. As is known in the art, the bearing in acruciate-substituting prosthesis includes a stabilizing post whichprojects superiorly from the articulating surface level. With the kneeloaded in a flexed position, such a stabilizing post experiences ananteriorly directed force due to the absence of the posterior cruciateligament. The force creates a moment about the transverse axis throughthe posts on the base that, if not counteracted, would cause theposterior edge of the bearing to lift off of the base. The base andposterior stabilized bearing must cooperate to withstand the load thatis normally carried by the posterior cruciate ligament, and the tibialbase as described above is so designed.

More specifically, the tibial component with the base and release memberas described above has a molded bearing retention force, measured interms of the maximum anteriorly directed force against a posteriorstabilized bearing that the tibial component can sustain withoutdissociation, in excess of 400 lbs., which is well above the maximumexpected in vivo load of 320 lbs. This retention force has been measuredin a worse-case scenario in which the base is made in the minimum sizepresently available in the Maxim® product line (59 mm) and the bearingis molded to the base with the maximum rated polyethylene thicknesspresently available in the Maxim® product line (24 mm), with nocompressive load. The test is performed by connecting a Maxim® femoralcomponent to the tibial component at 90° flexion and then loading thefemoral component axially to create an anterior force against thestabilizing post on the tibial bearing. The retention force would begreater with larger base sizes.

Referring now to FIG. 6, the molded primary bearing is removed byactuation of the release member with the aid of an extractor 60 having aball screw 62 enmeshed with a recirculating ball nut 64 that isnonrotatably mounted in a casing 66 having an axially extending prong 68mounted on the distal end thereof on either side of the ball screw as abrace for the extractor. Each prong 68 has a pointed tip, one of whichis partially cut away in FIG. 6 to reveal a portion of a threaded noseon a conical adapter, as will be described below. A suitable ball screwfor such purposes is commercially available from Thomson Saginaw BallScrew Company, Inc., Saginaw, Mich., as Catalog No. 720426SS. Acompatible ball nut is the Thomson Catalog No. 5708278 ball nut. Aprecision steel ball bushing bearing, Thomson Catalog No. A-81420, ismounted in the proximal end 70 of the casing to rotatably and slidablysupport an unthreaded proximal portion 72 of the ball screw shaft, and aT-handle 74 is mounted on that end of the ball screw shaft for rotationand consequent axial motion thereof. The tool preferably includes areleasable ratchet mechanism (not shown) to prevent clockwise rotationof the ball screw, thereby allowing the surgeon's grip on the T-handleto be temporarily released for repositioning during retraction of theball screw. The ball screw shaft also has an unthreaded distal portion76 on which is rotatably mounted an adapter 78 having an externallythreaded nose 80 designed to engage the threaded drive block 30 ofrelease member 12 in the tibial component. Access hole 50 in the bearingis formed with an inner diameter greater than the outer diameter of nose80 for this purpose.

In operation, the ball screw is advanced sufficiently to position thenose of the adapter axially beyond the prongs, and the nose of theadapter is then inserted through circular hole 50 in the anteriorsurface of the bearing and threaded into drive block 30 by manualrotation of the adapter. The ball screw is then retracted with the aidof the T-handle, without rotating the adapter. The prongs indent andthus firmly engage the anterior surface of the bearing as the ball screwis retracted, and, with the spacing between the bearing and ball nutfixed by the prongs, the release member is accordingly pulledanteriorly, whereupon the action of the wedge causes the bearing to movesuperiorly and disengage from the base. The primary bearing is raised tothe point of disengagement and then removed with the release member as aunit, as shown in FIG. 6. With the extractor as described above, themaximum translational force required to be exerted to remove a moldedprimary bearing from a 71 mm tray is approximately 670 pounds, whichcorresponds to a torque of approximately 30 inch-pounds applied to thehandle end of the ball screw shaft. This amount of torque is well withinthe capability of a surgeon or surgical assistant of normal strength.

Those skilled in the art will appreciate from FIG. 6 and the foregoingdescription that the translational force exerted by the extractor isapplied between the bearing and the release member, thereby avoiding anydisturbance to the base. The bearing is removed substantially in onepiece, although fragments of polyethylene may remain on the base, suchas in posterior grooves 54 of posts 22 and 24. Any such fragments areeasily removed, and the primary bearing is then replaced with a modularbearing and locking bar of the type described in the above-referencedU.S. Pat. No. 5,330,534.

Preferably the skirt on the molded primary bearing is trimmed aftermolding such that its inferior surface is not flush with the inferiorsurface 47 of the base, and, if desired, the skirt may be trimmed suchthat its inferior surface is flush with the superior surface of ledge45. When the tibial component is implanted, such trimming spaces thebearing apart from the tibial plateau and thus tends to minimize thechances of contact with bone cement, which is often applied to thetibial plateau to help secure the tibial base thereto and which may bepresent and exposed to some degree adjacent to the periphery of thebase. As a result, the primary bearing can be removed by actuation ofthe release member, as described above, without significant risk ofunwanted resistance from bone cement or of any removal or disturbance ofbone cement. As an alternative to such trimming, the mold may bemodified, if desired, to provide a correspondingly shorter skirt on thebearing.

The modular bearing has no skirt and is no wider than the tibial base,and is therefore slightly smaller than the primary bearing, whichextends slightly beyond the peripheral edge of the base as describedabove and as shown, e.g., in FIGS. 3 and 4. Alternatively, the modularbearing may be made as wide as the primary bearing it is designed toreplace, such that it likewise extends slightly beyond the peripheraledge of the base, albeit without a skirt.

A one-piece construction is preferred for the tibial base. The base maybe machined from titanium with an integral stem but is preferably castof cobalt-chromium. While a stem is a preferred part of the tibialcomponent, the principles of the invention also apply to a stemlesstibial base.

A first alternative embodiment of a nonmodular tibial componentaccording to the present invention is shown in FIGS. 7 and 8. In thisembodiment, a base 110 has an undercut portion 140 which is filled withpolyethylene during the molding process and which cooperates with othermolded bearing portions as in the preferred embodiment described aboveto secure the primary bearing on the tibial base. The undercut portionmay extend around the entire periphery of the base or selected portionsthereof. The construction of the tibial component, including releasemember 112, posterior rail 120, anterior posts 122 and 124, and moldedprimary bearing 126, is otherwise the same as that of the preferredembodiment described above. This embodiment may be useful in certainapplications, such as in uncemented tibial implants where there is nopossibility of contact with bone cement.

FIG. 9 is a perspective view of a second alternative embodiment of anonrmodular tibial component according to the present invention. In thisembodiment the release member 212 is provided with a pair of transversearms 213 and 214 on the anterior end of the drive block 230. Theposterior sides of the transverse arms fill the anterior grooves inposts 222 and 224 provided at the anterior edge of the base 210, and theanterior sides of the transverse arms are provided with respectivecutting ridges 215 and 216 each of which extends the length of its arm,as shown in FIG. 9. The cutting ridges are provided to facilitatepassage of the anterior end of the release member through the anteriorportion of the primary bearing 226 and thereby facilitate forward motionof the release member. A scalpel may be used during revision surgery tocut into or through the relatively thin layer of polyethylene(approximately 0.010-0.020″) covering the cutting ridge, if desired, tofurther facilitate the release action.

In this embodiment, tibial base 210 has a smooth, flat peripheral edge218 rather than a grooved or undercut edge as in the previousembodiments. Base 210 and the rail 220 and posts 222 and 224 integrallyformed thereon are otherwise the same as their counterparts in thepreferred embodiment, as is release member 212. Due to the smooth, flatedge and the transverse arms, this embodiment provides less retentionforce than the previous embodiments, which may be suitable in somecircumstances such as in cruciate-retaining applications. The retentionforce could be increased in this or either prior embodiment by formingthe release member without a tail that extends into the channel in therail. However, in this embodiment the anterior portion of the bearingalready tends to disengage from the base well before the posteriorportion. It is believed more desirable in most applications to have theanterior and posterior portions of the primary bearing disengage fromthe tibial base at approximately the same time, i.e., with negligibletilting, as in the preferred embodiment.

The present invention also encompasses other forms of retainers andother methods of removing a molded primary bearing without removing thebase. Other retainer forms presently contemplated as useful to varyingdegrees in certain applications include pegs, posts, holes or otherrecesses, dovetail configurations, tongue and groove configurations,mortise and tenon configurations, retaining walls, rails, detents, andsnap ring grooves.

For example, while the intercondylar posts and rail of the preferredembodiment constitute the preferred form of modular bearing retaineraccording to the present invention, the retainer may alternatively be inthe form of a raised flange or rim about other portions or all of theperiphery of the tibial base, with a cavity in an inwardly facingsurface of the flange or rim, wherein the modular bearing has acorresponding tongue or lip on its outer edge surface that fits into thecavity, as in U.S. Pat. No. 4,938,769 to Shaw or U.S. Pat. No. 4,795,468to Hodorek et al. Alternatively, or in addition as disclosed in thelatter patent, the modular bearing may be secured to the base with theaid of an auxiliary locking element in the form of a spring clip thatinterlocks a groove in the rim wall and a corresponding groove in theouter edge surface of the bearing. The clip is mounted in one groove soas to protrude therefrom and snap into the other groove when the modularbearing is pressed against the base. According to the present invention,the spring clip is preferably not incorporated in the primary tibialprosthesis, and the molded primary bearing preferably extends into thegroove in the rim.

As another alternative, the base may be provided with a threaded hole orholes designed to cooperate with a screw or screws to securely retain amodular bearing in place after the molded primary bearing has beenremoved during revision surgery. Such screws thereby serve as auxiliarymechanical locking elements. For example, a base with an internallythreaded stem designed to retain a modular bearing with the aid of ascrew, as disclosed in U.S. Pat. No. 4,959,071 to Brown et al., may bemodified to have an undercut peripheral edge and then placed in a moldadapted to provide a polyethylene skirt which covers the peripheral edgeand extends into the undercut portion sufficiently to secure the bearingto the base. The threaded stem hole is first plugged with a set screw orotherwise sealed to prevent entry of polyethylene, such as by coveringthe hole with a stopper of titanium or other suitable material towithstand the heat and pressure in the mold without deformation orfusing to the base.

The molded primary bearing in the alternative embodiment just describedis preferably removed in vivo by selectively cutting the skirt at theperipheral edge and then lifting the main portion of the bearing off ofthe base, using an osteotome if necessary or desired in conjunction witheither a preformed slot or a slot that is cut or melted into thepolyethylene at that time at the bearing-base interface to facilitateremoval. A witness mark may be provided on the bearing edge surface atthe time of molding to mark the level of the interface for suchpurposes. The set screw or stopper in the stem hole is removed after theprimary bearing, and a modular bearing is then placed on the base andsecured with a screw as described in the above-referenced Brown patent,which is hereby incorporated by reference along with theabove-referenced patents to Shaw and Hodorek et al. The removaltechnique just described may be facilitated by the use of a C-clamp withan osteotome mounted on one end or jaw, with the C-clamp connectedacross the bearing, e.g., in contact with the medial and lateral edgesof the bearing, with the tip of the osteotome inserted into a slotprovided for that purpose or aligned with a witness mark. The opposingforces exerted by the C-clamp do not disturb the tibial base. Thistechnique may also be used in lieu of a release member for removal of abearing molded to any of the tibial bases of FIGS. 1, 7 or 9.

Similarly, a pin may be placed through a coaxial pair of through holesin a modular bearing and base and held in position by a clip asdescribed in U.S. Pat. No. 5,061,271 to Van Zile, which patent is herebyincorporated by reference. The tibial base may have one or more dovetailsurface members such as disclosed in the above-referenced Van Zile orShaw patents.

The retainer may alternatively be designed to allow a modular bearing tofloat to some degree, either by translation or rotation. For example, amodular bearing may be rotatably mounted on a tibial base and retainedthereon by means of an unthreaded hole in the surface of the base incooperation with a mating stem on the inferior surface of the bearing.Peripheral edge grooves may be provided on the base and filled withpolyethylene, as described above with reference to FIG. 1, in order tosecure a nonmodular bearing to the base. Similar transversely orientedgrooves may be provided at a desired depth within the hole in the baseif desired. The nonmodular tibial prosthesis preferably includes acentrally mounted release member having a wedge and drive block similarto that shown in FIG. 1, with the wedge portion wider than the hole inthe base.

Also, other forms of release members are contemplated, includingone-piece and multi-piece mechanisms. For example, two wedges withcomplementary inclined surfaces could be provided with a counterdrivemechanism to force the wedges toward or away from each other, dependingon their orientation, so as to jointly raise the bearing off of thesuperior surface of the base.

In another alternative embodiment, the molded primary bearing is adaptedto be pulled directly away from the superior surface of the base withthe aid of a release member in the form of a simple plate, e.g., arelatively thin disc, located between the bearing and base. A modifiedform of the extractor shown in FIG. 6 is adapted to engage the platethrough an access hole provided in the superior surface of the bearingas molded. The plate has a threaded hole with its axis perpendicular tothe superior surface of the base, and is mounted in the intercondylararea of the base in place of the release member shown in FIG. 1. A#12-24 hole is suitable as in the preferred embodiment. A through holecoaxial with the hole in the plate is provided in the superior surfaceof the bearing as an access hole for a threaded nose of an extractor asshown in FIG. 6 but modified to have a longer nose for this purpose. Theextractor is adapted to be aligned with the access hole axis with afixed spacing between the extractor casing and the tibial base. Thisspacing may be maintained with prongs as shown in FIG. 6 but spacedfurther apart so as to straddle the bearing and engage contact pointsprovided on the base for this purpose. The contact points may, forexample, be peripheral extensions of ledge 45 on the base of FIG. 1which are not covered with polyethylene during the molding process. Ifthe prongs have pointed tips as in the embodiment of FIG. 6, eachcontact point on the base preferably has an indentation in its superiorsurface to receive a prong tip and fix its position in the transverseplane. The base is otherwise the same as shown in FIG. 1.

In operation, the extractor is positioned with the prongs on the contactpoints on the base and the extended nose of the extractor threaded intothe plate. The ball screw of the extractor is then retracted, wherebythe plate exerts a superiorly directed force against the bearing andthus raises it off of the base surface. For this purpose, the shear areaof the portion of the molded bearing above the plate is greater than thetotal shear area of the portions of the molded bearing in the grooves inthe posts, rail and peripheral edges of the base.

As an alternative to prongs of the type shown in FIG. 6, the extractormay be provided with a pair of jaws with opposed sharp edges adapted tocut into the peripheral skirt on the bearing and engage themedial/lateral edge grooves 42 in the tibial base, whereby the extractormay be clamped to the base. With the nose of the extractor threaded intothe plate, the primary bearing may be pulled directly away from thesuperior surface of the base along with the release plate uponretraction of the ball screw in the extractor.

Alternatively, the primary bearing may be removed in vivo by sectioning,reaming or grinding the bearing or key points thereof. As one example ofsuch a technique in the context of a hip joint prosthesis, an acetabularcup having a porous shell may be provided with an annular groove forretention of a preformed bearing insert as a modular revision bearing,and the primary bearing may be formed in the porous shell by directcompression molding of polyethylene which is later reamed out duringrevision surgery sufficiently to allow insertion of the preformedinsert. The shell may be either a metal shell with a porous coating oran uncoated shell made of a porous material having structural integritysuch as porous tantalum. An acetabular & cup with an interior annulargroove, in a nonporous metal shell, is disclosed in U.S. Pat. No.5,571,201 to Averill et al., which patent is hereby incorporated byreference. That groove is designed to receive cement, but,alternatively, a similar groove may be designed and constructed toreceive a snap ring for mechanical retention of a bearing insertprovided with a corresponding groove for the snap ring. The groove inthe cup is preferably masked off prior to the molding process with aremovable annular plug of a material that will not fuse to thepolyethylene or shell during the molding process. During revisionsurgery, a surgeon would ream the primary bearing down to the surface ofthe porous shell, remove the annular plug, and then insert a modularbearing insert.

A bearing of the type shown in FIG. 2 but molded to the base 10 withouta release member thereon may alternatively be removed by sectioning. Forexample, a medial-lateral strip ¼″ to ½″ wide may be cut out of thebearing with a saw or mill, and the remaining anterior and posteriorbearing portions may then be forced toward each other, and therebydisengaged from the posts, the rail and the edge grooves on the base,with the aid of pliers or a clamp adapted with prongs or otherwise topierce or otherwise firmly engage the anterior and posterior bearingsurfaces.

Reamers, saws and osteotomes may disturb a prosthesis or damage it,e.g., by scratching the smooth superior surface of a tibial base.Therefore, while methods involving such tools are options for revisionprocedures, it is preferred to incorporate a release member in thenonmodular prosthesis which allows a bearing to be removed without theapplication of force to the base member.

It is also contemplated that the nonmodular bearing may be secured inways other than by molding in certain applications. For example, aninterference fit may be provided between a preformed bearing and amating base with the aid of a press in the factory. Thermally aidedassembly is another alternative. For example, a barbed central post onthe base may be heated so as to locally melt the polyethylene in apreformed bearing as the bearing is pressed onto it; upon cooling, thepolyethylene is secured to the post and thereby to the base, which maybe provided with ancillary retaining elements such as a retaining rim. Anonmodular prosthesis may also be fabricated using a permanent adhesiveto secure a preformed bearing to a base. Such methods are consideredpossible although not preferred. A recess must be provided in thebearing's inferior surface for a release member if one is desired forthe intended application.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly the preferred embodiment has been shown and described and that allchanges and modifications that come within the spirit of the inventionare desired to be protected.

What is claimed is:
 1. A hip joint prosthesis convertible in vivo to amodular device, comprising an acetabular cup having a modular bearingretainer, and a primary bearing directly molded to said acetabular cup.2. The hip joint prosthesis of claim 1, wherein said modular bearingretainer is adapted to cooperate with an auxiliary mechanical lockingelement to securely retain a modular bearing in said acetabular cup. 3.A method of replacing a hip joint, comprising: providing an acetabularcup with a retainer for a modular replacement bearing to be secured tosaid acetabular cup after removal of a primary bearing therefrom;molding a primary bearing to said acetabular cup; and implanting saidacetabular cup with said retainer and molded primary bearing in apatient during primary hip replacement surgery.
 4. The method of claim3, further comprising: removing said molded primary bearing duringrevision surgery without removing said acetabular cup; and securing amodular bearing to said retainer during said revision surgery.